The present invention relates generally to the field of olefin polymerization catalysis, catalyst compositions, methods for the polymerization and copolymerization of olefins, and polyolefins. More specifically, this invention relates to half-metallocene compounds with a heteroatom-containing ligand bound to the transition metal, and catalyst compositions employing such hybrid compounds.
Polyolefins such as high density polyethylene (HDPE) homopolymer and linear low density polyethylene (LLDPE) copolymer can be produced using various combinations of catalyst systems and polymerization processes. One method that can be used to produce such polyolefins employs a chromium-based catalyst system. HDPE and LLDPE resins produced using a chromium-based catalyst system generally have a broad molecular weight distribution. For instance, resins having a polydispersity index (PDI) greater than 6 are not unusual. Polyolefin resins produced using a chromium catalyst also can have a low level of long chain branching. This combination of properties is difficult to duplicate with other commercially viable catalyst systems. Metallocene catalysts, for example, generally produce polyolefins with a much narrower molecular weight distribution and either have too little, or too much, long chain branching. Likewise, Ziegler-type catalyst systems produce polyolefin resins which are typically much narrower in molecular weight distribution and have substantially no long chain branching. Polyolefin resins produced using a ballard catalyst are generally too high in molecular weight, too broad in molecular weight distribution, and contain too much long chain branching.
It would be beneficial to have a non-chromium catalyst system that could produce a polyolefin homopolymer or copolymer having the desired combination of a relatively broad molecular weight distribution and a relatively low level of long chain branching. Accordingly, it is to this end that the present invention is directed.